The present invention relates to the general field using composite material to make annular structural parts for a turbine engine, and more particularly to retention casings for gas turbine fans of aeroengines.
In a gas turbine aeroengine, a fan casing performs several functions: it defines the air inlet passage into the engine; it supports an abradable material facing the tips of the fan blades; it supports an optional structure for absorbing soundwaves in order to treat noise at the inlet to the engine; and it incorporates or supports a retention shield. Such a shield constitutes a trap for retaining debris, such as ingested objects or fragments of damaged blades that are projected outwards by centrifuging, in order to prevent them passing through the casing and reaching other parts of the aircraft.
Proposals already exist to make a fan retention casing out of composite material. By way of example, reference may be made to Document EP 1 961 923, which describes fabricating a casing out of composite material, the thickness of the casing varying and including the formation of fiber reinforcement constituted by superposed layers of a fiber texture with the fiber texture being densified by a matrix.
Document WO 2012/140355 describes an example of a winder machine suitable for use for performing such a method of fabricating a fan retention casing out of composite material. That machine comprises in particular a takeup mandrel that stores the fiber texture and a mandrel of an injection mold (referred to below as the impregnation mandrel) onto which the fiber texture stored on the takeup mandrel is to be transferred, the impregnation mandrel having an outside profile that corresponds to the inside profile of the casing that is to be fabricated.
Such a winder machine also includes a unit for controlling motors for driving the mandrels in rotation and serving to monitor the tension applied to the fiber texture while it is being wound on the impregnation mandrel. By monitoring this winding tension, and depending on the nature of the fiber texture, it is possible to determine and control the fiber content of the resulting preform.
At the end of the winding operation, compacting covers are fastened on the impregnation mandrel so as to contain the fiber preform in a cavity. The preform is then impregnated with resin under pressure so that, after the resin has been polymerized, the final shape of the fan retention casing is obtained.
When closing the injection mold by means of the covers, the fiber preform is compacted. Because of the expansion of the fibers, the preform occupies a greater volume in its free state than when it is confined inside the cavity of the injection mold.
It has been found necessary to control the expansion of the preform during winding of the fiber texture on the impregnation mandrel. If this expansion is too great, there is a risk of generating wrinkles when closing the injection mold. Conversely, if the expansion is too small, there is a risk of the fiber content in the resulting preform being too low (as a result of over-compacting).
It is also known that the expansion of the preform is associated directly with knowledge of the thickness of the fiber texture that is wound on the impregnation mandrel. Controlling expansion of the preform thus requires obtaining knowledge about the thickness of the fiber texture that is wound on the impregnation mandrel.
Unfortunately, the techniques presently known for measuring the thickness of the fiber texture on the impregnation mandrel are not satisfactory. In particular, one of the known solutions consists in measuring the preform manually by means of a caliper, e.g. once every half-turn during the winding operation. Such a solution presents numerous drawbacks. Taking such measurements is lengthy, awkward, relatively inaccurate, and depends on an operator. In addition, that measurement technique requires the winding operation to be stopped frequently, and such stops may amount to 40% of the time of the winding operation.